Solar cells are semiconductor devices for converting optical energy to electric power. Among others, monocrystalline silicon solar cells become the mainstream solar cells of widespread use because they have high conversion efficiency and are relatively easy to manufacture. For the monocrystalline silicon solar cells, it is a common practice to form microscopic protrusions, known as texture, on the surface for the purpose of preventing a reflection loss, as disclosed in JP-A H09-129907 (Patent Document 1) and JP-A H10-7493 (Patent Document 2), for example. If the surface of a solar cell is flat, a part of incident light is reflected and lost without being converted to electric current. By contrast, the texture structure offers a chance for part of the reflected light to reenter the solar cell plural times. Since the solar cell is consequently reduced in reflectivity of the light-receiving surface, short-circuit current is mitigated, and the solar cell performance is significantly improved.
As disclosed in Patent Documents 1 and 2, the texture structure mentioned above is formed by anisotropic etching of a monocrystalline silicon substrate. Anisotropic etching makes use of a difference in etch rate dependent on silicon plane orientation. Specifically, a monocrystalline silicon substrate is prepared by slicing with a wire saw or similar tool so that the substrate has a work damaged layer originating from the slicing step. After the damaged layer is etched away, anisotropic etching is carried out by immersing the substrate again in a hot alkaline aqueous solution of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate or sodium hydrogencarbonate. Notably, an amount of 2-propanol is often dissolved in the alkaline aqueous solution to promote the reaction.
The urgent requirements currently posed on the solar cell include an increase of photoelectric conversion efficiency and a cost reduction by simplification of the solar cell manufacture process. On the surface of the substrate from which the damaged layer has been etched away, sometimes abrasive grains and contaminants from the slurry and wire saw used in the slicing step are left in minute amount. It is very difficult to exclude traces of contaminants even if such contaminants are washed away with a surfactant or the like.
Besides, heavy metals and other contaminants adhere to the substrate during the steps of forming p-n junction, forming antireflective coating, and forming front and back side electrodes and the like. It is difficult to remove these contaminants completely even if the substrate is cleaned with a mixed aqueous solution of hydrochloric acid and aqueous hydrogen peroxide, for example. They cause a reduced bulk lifetime and become a barrier to efficient solar cells.
It is noted that a method of manufacturing a solar cell is known in the art as described in JP-A 2005-209726 (Patent Document 3). The solar cell is manufactured by slicing a monocrystalline silicon ingot to form a monocrystalline silicon substrate, the monocrystalline silicon substrate including a primary damaged layer formed by slicing work on a first major surface side, removing the primary damaged layer mechanically or chemically, inflicting a new damage by another mechanical work other than the slicing work, to form a secondary damaged layer thinner than the primary damaged layer, anisotropically etching the secondary damaged layer to form a texture structure, and forming a light-receiving side electrode on the texture structure.